Two step process for polymerizing ethylene

In a process for polymerizing ethylene alone or ethylene in admixture with a small amount of another .alpha.-olefin or conjugated diolefin in two successive polymerization steps, the improvement comprising:

• polymerizing 30 to 70 wt.% of said ethylene or ethylene mixture in the presence of hydrogen at a molar ratio of ethylene or ethylene mixture to hydrogen of 1:1-8 in the gaseous phase of the polymerization system in the first step and polymerizing the residual amount of said ethylene or ethylene mixture in the second step at an ethylene or ethylene mixture:hydrogen molar ratio of 1:0-0.3 in the gaseous phase of the polymerization system, wherein the catalyst in each of said polymerization steps comprises the combination of (1) an organoaluminum compound (C) selected from the group consisting of organometallic compounds sufficient for preparing Ziegler type catalysts and (2) a specific solid catalyst component (B) obtained by reacting (II) a titanium or vanadium halogen containing compound with a reaction product (A) obtained by reacting (I) a Grignard reagent with a silicon compound selected from the group consisting of a hydropolysiloxane having the formula:
• ##EQU4## wherein R' is a monovalent alkyl, aryl, aralkyl, alkoxy or aryloxy group, a is 0,1 or 2,b is 1, 2 or 3 and a+b.ltoreq.3 and a silicon compound including the formula R.sub.n.sup.2 Si(OH).sub.4-n wherein R.sup.2 represents a C.sub.1 -C.sub.18 monovalent hydrocarbon moiety and n is 1, 2 or 3, said reacting (II) being in the presence or absence of an aluminum alkoxide, an aluminum alkoxyhalide or the reaction product obtained by reacting an alkoxyaluminum compound with water.